1. Field of the Invention
The present invention generally relates to a power conversion field, in particular, to a non-isolated resonant converter applied to photovoltaic inverters, LED constant-current driving and multi-path LED constant-current driving.
2. Description of Related Art
High frequency, high efficiency and high power density are the developing trends of switching power supply. By the characteristics of soft switching and great EMI performance, the resonant converter becomes important research issues in the power conversion technology field.
Most of the common resonant converter is formed based on the isolated transformer. Referring to FIG. 1 to FIG. 3, each of the resonant half-bridge converters respectively shown in FIGS. 1-3 is implemented by using the isolated transformer. The output of the resonant converters shown in FIG. 1 and FIG. 2 is implemented by using the full-wave rectification circuit, and the output of the resonant converter shown in FIG. 3 is implemented by using the half-wave double rectification circuit. To be specific, for FIG. 3, when the voltage of the common-polarity terminal (i.e. the dotted terminal) of the vice-side (secondary) winding of the isolated transformer is positive, the vice-side (secondary) winding of the isolated transformer supplies power to the load through a capacitor C and a diode D1 both connected with the vice-side winding of the isolated transformer, wherein the capacitor C has a function of voltage boosting. On the other hand, when the voltage of the common-polarity terminal of the vice-side winding of the isolated transformer is negative, the diode D1 is in the cut-off state, a diode D2 is in the conducting state, such that the capacitor C is charged at this time.
The duty cycle of a half-bridge switch circuit, which is constituted by two series-connected switch-transistors, is 50%, i.e., TON=TOFF. Assuming that the average voltage of the two terminals of the capacitor C is Vc, so according to the voltage-second balance characteristic, it can be obtained the following equation:(VO−VC)×TON=VC×TOFF.
Therefore,
      V    C    =                    V        O            2        .  
Moreover, as shown in FIG. 14, a circuit diagram of an existing isolated resonant symmetric half-bridge converter is illustrated. A capacitor C141 and a capacitor C142 are connected in series. A switch-transistor Q1 and a switch-transistor Q2 are connected in series. A branch formed by the capacitors C141, C142 and a branch formed by the switch-transistors Q1, Q2 are connected in parallel and are both connected in parallel with the power (i.e. Vin). The negative electrode of the power (Vin) is grounded (or connected to a ground potential), and the parameters of the capacitors C141 and C142 are the same. The common-polarity input terminal (i.e. the dotted input terminal) of the isolated transformer is connected between the capacitors C141 and C142 (i.e., terminal 1); and the opposite-polarity input terminal (i.e. the non-dotted input terminal) of the isolated transformer is connected between the switch-transistors Q1 and Q2 (i.e., terminal 2) through the capacitor C143 and inductor L14. The vice-side winding of the isolated transformer is connected with a full-wave double synchronous rectifying (SR) circuit. The full-wave double synchronous rectifying circuit includes switch-transistor Q3, Q4 and capacitors C144, C145. The switch-transistors Q3 and Q4 are connected in series. The capacitors C144 and C145 are connected in series. A branch formed by the switch-transistors Q3, Q4 and a branch formed by the capacitors C144, C145 are connected in parallel, and two terminals of the parallel-connected branches are respectively served as a positive electrode and a negative electrode of the output of isolated resonant symmetric half-bridge converter. The common-polarity output terminal (i.e. the dotted output terminal) of the isolated transformer is connected between the capacitors C144 and C145. The opposite-polarity output terminal (i.e. the non-dotted output terminal) of the isolated transformer is connected between the switch-transistors Q3 and Q4. Based on the configuration of FIG. 14, the input of the isolated transformer is reduced due to the voltage difference between terminals 1 and 2 is Vin/2.
Since the isolated transformer is constituted by two independent windings, so the size and the loss of the resonant converter become larger and higher, which are important issues to be improved.